1. Field of the Invention
The present invention relates generally to a method for automatically adjusting the interval of time between defrost cycles in a heat pump system. The method utilizes measurements of the duration of the previous defrost cycle or cycles, and adjusts the time interval before initiating the next defrost cycle so that any frost build-up can be defrosted without unnecessary defrost cycles.
2. Description of the Related Art
Heat pump systems generally build frost on the outdoor heat exchanger coil when operating in the heating mode. This frost build-up can gradually degrade the heat exchanger and system performance in the form of heating capacity and efficiency. If the frost is not cleared, it can continue to build-up until the heat exchanger coil becomes completely blocked with ice. At this point, in most heat pump systems, protective devices typically cause the system to shut down. If the protective devices are not effective, equipment failure could occur.
For these reasons, it is common practice in most heat pump systems to incorporate a way to defrost. For example, most heat pump systems operate in the cooling (air conditioning) mode for short periods of time, thereby reversing the flow of refrigerant in the system with the help of a reversing valve. Also, during this defrost cycle, the outdoor fan, which blows air over the outdoor heat exchanger coil, is stopped. When the heat pump operates in the cooling mode without the outdoor fan running, the outdoor heat exchanger coil heats up quickly, to melt the frost.
Defrosting in this manner has its penalties. Running the heat pump in cooling mode while the home needs heating capacity clearly leads to wasted energy. Furthermore, the cold air delivered inside the home can be quite uncomfortable in the heating season. To warm up the air to comfortable levels during a defrost cycle, most heat pump systems activate a supplemental heat source. Typically, this supplemental source is electric strip heat, which itself consumes a great amount of electric energy. Another problem is that two refrigerant flow reversals are needed in a defrost cycle, from heating to cooling and back to heating. The flow reversals are usually quite noisy, and are an annoyance to the consumer.
In order to minimize the negative impact of these defrost cycles on energy usage, noise levels, and consumer comfort, it is desirable to reduce the frequency and duration of defrost cycles. The ideal system should defrost just often enough to eliminate frost and no more. This can be quite challenging because the rate of frost build up varies with the weather. Outdoor temperatures, humidity and wind levels all play a role in determining how much frost accumulates on the coils. Different climatic areas have different weather patterns and, therefore, different defrost requirements.
Several defrost control strategies and algorithms have been employed in the prior art. Most defrost controls involve use of electronic circuits or microprocessors. The general approach is to estimate the presence of sufficient frost to initiate a defrost cycle, and then determine a sufficient clearing of frost from the coils to terminate the defrost cycle.
The most common method of terminating defrost cycles involves sensing the temperature at an appropriate point in the heat exchanger coil. During a defrost cycle, the coil starts to heat up as the hot compressed refrigerant flows through it. However, the heat is first used to melt whatever amount of frost there is on the coil. Once all the frost is cleared, the heat starts to increase the temperature of the coil very quickly. A defrost control that has a coil temperature sensor can detect this increased temperature and terminate the defrost cycle. Alternately, a pressure sensor or pressure switch can be used. Some simple defrost controls have no sensors, and instead run all defrost cycles for a fixed duration.
Determining when to initiate a defrost cycle is more challenging. A number of methods are employed in the prior art. These methods generally fall into two categories: “demand” defrosts and “timed” defrosts. Demand defrosts attempt to estimate the actual frost level or rate of frost accumulation under any set of conditions and wait until this estimate indicates a “demand” for defrosting before initiating. Since there is no practical direct sensing of frost level, these demand defrost methods use surrogate sensors to provide an estimate of the frost level. One example is to use the difference between coil temperature and outdoor air temperature. In this method, when the coil temperature falls sufficiently below the outdoor air temperature, a defrost cycle is initiated. The applicable principle is that a relatively colder coil will accumulate more frost. Many other schemes with varying degrees of sophistication have been used. These methods are not completely foolproof. They may defrost too frequently or too infrequently. The consequences are either a “block of ice” on the coils or complaints by consumers about too many defrosts.
The alternative, a timed defrost method, is much simpler. The control simply has a fixed time interval between defrost cycles. Typically, this time interval is in terms of accumulated heating mode operating time, not just elapsed time. Also, the installer of the heat pump system can typically select this fixed time interval from several available choices such as: 30 minutes, 60 minutes, 90 minutes, and 120 minutes. Once selected, the fixed time interval is applied for the life of the product, unless changed again by a service technician. Typically the defrost interval is selected by installing technicians to suit, in their judgment, the climatic conditions in their area.
As mentioned above, these conditions can change dynamically with the weather. A fixed “timed” defrost interval cannot always match the current defrost needs. This can lead to the same problems as “demand” defrosts.
While both methods described above have been extensively used, there is a desire for improvement.